U.S. patent number 5,232,600 [Application Number 07/965,538] was granted by the patent office on 1993-08-03 for hydrophobic membranes.
This patent grant is currently assigned to Pall Corporation. Invention is credited to Peter J. Degen, Thomas C. Gsell, Isaac Rothman.
United States Patent |
5,232,600 |
Degen , et al. |
August 3, 1993 |
**Please see images for:
( Certificate of Correction ) ** |
Hydrophobic membranes
Abstract
A process for the preparation of a microporous polymeric
membrane is provided which includes polymerizing a polymerizable
fluorine-containing monomer in the presence of a microporous
polymeric membrane substrate such that the fluorine-containing
monomer forms a polymeric superstrate that is permanently
chemically bonded to all surfaces of the membrane. A device is also
provided for processing fluids which incorporates a microporous
polymeric membrane having a CWST of less than about 28 dynes/cm and
includes a microporous polymeric membrane substrate formed from a
material such as a fluoropolymer and, permanently chemically bonded
to all portions of the surface thereof is a superstrate, which may
be a polymer or copolymer derived from a monomer(s), having an
ethylenically unsaturated group and a perfluoroalkyl group. A gas
filtration/drying process, a method of venting a gas from a vessel
and a method of separating a gas from a liquid in a gas/liquid
mixture using a microporous polymeric membrane having a CWST less
than about 28 dynes/cm which includes a microporous polymeric
membrane substrate and a superstrate fluoropolymer permanently
chemically bonded to all portions of the membrane substrate are
also provided.
Inventors: |
Degen; Peter J. (Huntington,
NY), Rothman; Isaac (Brooklyn, NY), Gsell; Thomas C.
(Glen Cove, NY) |
Assignee: |
Pall Corporation (Glen Cove,
NY)
|
Family
ID: |
27407987 |
Appl.
No.: |
07/965,538 |
Filed: |
October 23, 1992 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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537793 |
Jun 14, 1990 |
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351219 |
May 15, 1989 |
4954256 |
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Current U.S.
Class: |
210/640; 210/490;
210/500.35; 210/500.4 |
Current CPC
Class: |
B01D
19/0031 (20130101); C08J 9/405 (20130101); B01D
67/0093 (20130101) |
Current International
Class: |
B01D
67/00 (20060101); B01D 19/00 (20060101); C08J
9/00 (20060101); C08J 9/40 (20060101); B01D
071/34 () |
Field of
Search: |
;55/16,158
;210/640,651,652,490,500.35,500.36,500.42,500.41 ;421/245
;521/31,53,137,134 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0057065 |
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Apr 1982 |
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EP |
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0144054 |
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Dec 1985 |
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EP |
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0216622 |
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Apr 1987 |
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EP |
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2585025 |
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Jan 1987 |
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FR |
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2608452 |
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Jun 1988 |
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FR |
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2014184A |
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Aug 1979 |
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GB |
|
Primary Examiner: Spear; Frank
Attorney, Agent or Firm: Leydig, Voit & Mayer
Parent Case Text
This application is a continuation of application Ser. No.
07/537,793, filed Jun. 14, 1990, now abandoned, which is a division
of application Ser. No. 07/351,219 filed May 15, 1989, now U.S.
Pat. No. 4,454,256.
Claims
We claim:
1. A process for the preparation of a microporous polymeric
membrane which comprises polymerizing a polymerizable
fluorine-containing monomer in the presence of a microporous
polymeric membrane substrate such that the fluorine-containing
monomer forms a polymeric superstrate that is permanently
chemically bonded to all surfaces of the membrane.
2. A process according to claim 1 in which the membrane is
contacted with a solution of the fluorine-containing monomer which
is then subjected to polymerization conditions.
3. A process according to claim 1 in which the polymerizable
fluorine-containing monomer is polymerized in the presence of a
non-fluorine-containing ethylenically unsaturated monomer so as to
form a copolymer that is permanently chemically bonded to the
surfaces of the membrane.
4. A process according to claim 1 in which the polymerization is
effected under the influence of ionizing radiation.
5. A process according to claim 4 in which the ionizing radiation
is .gamma.-radiation.
6. A process according to claim 5 in which an ionizing radiation
dose rate of from about 5 to about 100 krad/hr is used.
7. A process according to claim 6 in which the radiation dose rate
is from about 5 to about 15 krad/hr.
8. A process according to claim 4 in which the ionizing radiation
is provided by a .sup.60 Co source.
9. A process according to claim 1 in which the microporous
polymeric membrane is formed from a fluorine-containing
polymer.
10. A process according to claim 9 wherein said fluorine-containing
polymer is poly(vinylidene fluoride).
11. A process according to claim 1 in which the microporous
polymeric membrane is formed from a polyamide.
12. A process according to claim 1 wherein said microporous
polymeric membrane substrate is formed from a material selected
from the group consisting of polyolefins, polyamides, polyesters,
polyurethanes, polysulfones, poly(vinylidene fluoride);
polytetrafluoroethylene and perfluoroalkoxy resins.
13. A process according to claim 1 wherein said microporous
polymeric membrane substrate comprises a hydrophobic material
having a CWST of less than about 35 dynes/cm.
14. A device for processing fluids which incorporates a microporous
polymeric membrane having a CWST of less than about 28 dynes/cm and
comprising a microporous polymeric membrane substrate and,
permanently chemically bonded to all portions of the surface
thereof, a superstrate fluoropolymer.
15. A device according to claim 14 wherein said microporous
polymeric membrane substrate is formed from a material selected
from the group consisting of polyolefins, polyamides, polyesters,
polyurethanes, polysulfones,
16. A device for processing fluids which incorporates a microporous
polymeric membrane having a CWST of less than about 28 dynes/cm and
comprising a microporous, fluoropolymeric membrane substrate and,
permanently chemically bonded to all portions of the surface
thereof, a superstrate (co)polymer of a monomer having an
ethylenically unsaturated group and a perfluoroalkyl group.
17. A gas filtration/drying process which comprises passing a gas
through a microporous polymeric membrane having a CWST less than
about 28 dynes/cm comprising a microporous polymeric membrane
substrate and permanently chemically bonded to all portions of the
surface thereof, a superstrate fluoropolymer.
18. A method of venting a gas from a vessel which comprises
allowing the gas to vent through a microporous polymeric membrane
having a CWST less than about 28 dynes/cm comprising a microporous
polymeric membrane substrate and permanently chemically bonded to
all portions of the surface thereof, a superstrate
fluoropolymer.
19. A method of venting a gas from a vessel according to claim 18
wherein said microporous polymeric membrane substrate is formed
from a material selected from the group consisting of polyolefins,
polyamides, polyesters, polyurethanes, polysulfones,
poly(vinylidene fluoride); polytetrafluoroethylene and
perfluoroalkoxy resins.
20. A method of venting a gas from a vessel according to claim 18
wherein said microporous polymeric membrane substrate is
poly(vinylidene fluoride).
21. A method of venting a gas from a vessel according to claim 18
wherein said superstrate fluoropolymer comprises a
fluoroalkanesulfonamidoethyl acrylate.
22. A method of venting a gas from a vessel according to claim 21
wherein said superstrate fluoropolymer comprises a
2-(n-ethylperfluorooctanesulfonamido)ethyl acrylate.
23. A gas filtration/drying process according to claim 17 wherein
said microporous polymeric membrane substrate is formed from a
material selected from the group consisting of polyolefins,
polyamides, polyesters, polyurethanes, polysulfones,
poly(vinylidene fluoride); polytetrafluoroethylene and
perfluoroalkoxy resins.
24. A gas filtration/drying process according to claim 17 wherein
said microporous polymeric membrane substrate is poly(vinylidene
fluoride).
25. A gas filtration/drying process according to claim 17 wherein
said superstrate fluoropolymer comprises a
fluoroalkanesulfonamidoethyl acrylate.
26. A gas filtration/drying process according to claim 25 wherein
said superstrate fluoropolymer comprises a
2-(N-ethylperfluorooctanesulfonamido)ethyl acrylate.
27. A method of separating a gas from a liquid in a gas/liquid
mixture which comprises allowing the gas to pass through a
microporous polymeric membrane having a CWST less than about 28
dynes/cm comprising a microporous polymeric membrane substrate and
permanently chemically bonded to all portions of the surface
thereof, a superstrate fluoropolymer.
28. A method of separating a gas from a liquid in a gas/liquid
mixture according to claim 27 wherein said microporous polymeric
membrane substrate is formed from a material selected from the
group consisting of polyolefins, polyamides, polyesters,
polyurethanes, polysulfones, poly(vinylidene fluoride);
polytetrafluoroethylene and and perfluoroalkoxy resins.
29. A method of separating a gas from a liquid in a gas/liquid
mixture according to claim 27 wherein said microporous polymeric
membrane substrate is poly(vinylidene fluoride).
30. A method of separating a gas from a liquid in a gas/liquid
mixture according to claim 27 wherein said superstrate
fluoropolymer comprises a fluoroalkanesulfonamidoethyl
acrylate.
31. A method of separating a gas from a liquid in a gas/liquid
mixture according to claim 30 wherein said superstrate
fluoropolymer comprises a
2-(N-ethylperfluorooctanesulfonamido)ethyl acrylate.
Description
TECHNICAL FIELD
This invention relates to a hydrophobic microporous membrane whose
wetting characteristics can be controlled so that the maximum
surface tension of liquids which will wet the membrane is less than
about 28 dynes/cm. This invention also relates to a method for
making such membranes.
BACKGROUND OF THE INVENTION
Microporous membranes, i.e., thin sheets of material having pores
from a few micrometers in diameter down to about 0.05 .mu.m in
diameter, have long been known. Such membranes may be made out of
many different materials such as naturally occurring polymers,
synthetic polymers, and ceramics. Depending upon the material from
which the membrane is made, its wetting characteristics may differ
greatly.
Liquid-repelling membranes often find use in filtration of gases,
venting filters, and gas vents. Such membranes are herein referred
to as "hydrophobic" even though, as will be clear from the context,
liquids other than water (surface tension about 72.4 dynes/cm) are
repelled by such membranes. Hydrophobic membranes are effective in
these applications because they will allow gases and vapors, which
have low surface tensions, to pass through the membrane while
excluding materials with high surface tensions, for example, many
liquids, from the membrane. For example, a gas filter will be
effective if it allows only gas to pass but will not allow drops of
liquid such as steam condensate, pump oil droplets, or other mists
to penetrate and fill (and thereby block) the pores of the
filter.
Frequently, these situations are encountered by filters used to
sterilize the air feed to a biological fermentor. These filters are
often sterilized after installation by exposure to steam. Should
steam condensate penetrate and remain in the filter membrane the
membrane would become blocked to further steam and subsequent air
flow during use. Similarly, if water or oil droplets from air
compressors or other sources should penetrate the filter membrane
during use, the membrane would become blocked and reduce air flow
during further use.
Hydrophobic membranes are also used in vent filters. In this
application they protect the cleanliness of a liquid inside a
vessel while permitting the vapor in the head space of that vessel
to flow freely, both into and out of the vessel, as that vessel is
filled and/or depleted of its contents. It frequently occurs that
the liquid in such a vented vessel contacts the filter membrane in
vent filters due to splashing or overfilling the vessel. If the
membrane is wetted upon contact with the liquid, the liquid will
penetrate the membrane and fill its pores, eliminating free flow of
gas through the filter. Restrictions in flow through the vent will
cause reduced drainage of liquid from the vessel and in some
instances, collapse of the vessel itself. To perform effectively in
such applications, the membrane must not be wetted by the liquid
upon contact with it.
Hydrophobic membranes are also used in gas venting applications
where the membrane is in constant contact with a liquid containing
bubbles of gas. In such applications the membrane must serve as a
barrier to the liquid and contain it while permitting the gas in
the liquid to escape through the membrane. The membrane also serves
as a filter to protect the contained liquid from contamination from
the environment to which the gas escapes. In such applications,
too, the membrane must not be wetted by the liquid upon contact
with it. If the liquid were to wet the membrane, the liquid would
penetrate the membrane, flow through it, and be lost from its
contained system. Furthermore, the membrane would then be blocked
by the liquid and no longer will be permeable to gas. It would then
be unable to function as a vent.
In many of the above applications the membrane must behave as a
sterilizing barrier; that is, it must be completely bacterially
retentive. Not only must the membrane itself have such a small pore
size that it can perform such a function, but also the device must
be completely sealed so that it will not leak or bypass. To qualify
for use in such critical applications it must be possible to test
the device in order to determine that there are no faults and to
ensure its ability to function.
Most frequently this is done by means of tests such as "bubble
point" or "pressure hold" tests. These tests are referred to as
integrity tests and are well known to those skilled in the art.
They make use of the capillary properties of microporous membranes
when fully wetted with a suitable test liquid. To be effective in
such gas and vent filter applications, the filter membrane of
choice must completely reject the liquids with which it may come in
contact during use. However, it must also be able to be fully
wetted by a suitable liquid used for testing the integrity of the
filter or device. The wetting characteristics of the membrane must
therefore be controlled carefully so that the membrane will not be
wetted by most liquids encountered during fluid handling operations
yet will be easily and completely wetted by special fluids used for
carrying out integrity tests.
The ability of a solid surface to be wetted upon contact with a
liquid depends upon the surface tension of the liquid and the
surface free energy of the solid surface. In general, if the
surface tension of the liquid is less than the surface free energy
of the solid surface, the surface will be spontaneously wetted by
that liquid. An empirical wetting property of a porous matrix, its
critical wetting surface tension (CWST), can easily be determined.
The CWST of a porous matrix such as a microporous membrane may be
determined by finding the liquid having the highest surface tension
within a homologous series of inert liquids which will
spontaneously wet the porous matrix. For the purposes of this
disclosure a porous membrane being "spontaneously wetted" means
that when such a membrane is placed in contact with a liquid that
liquid is drawn into the porous structure of the membrane within a
few seconds without the application of external pressure. Liquids
having surface tensions below the CWST of the porous matrix will
wet it; liquids having surface tensions above the CWST of the
porous matrix will not wet it and will be excluded.
Membranes made of materials which contain only non-polar groups and
which have low critical surface tensions are not spontaneously
wetted by liquids having high surface tensions, for example, water
and most aqueous solutions. Microporous membranes made of non-polar
materials such as polypropylene, poly(vinylidene fluoride), and
polytetrafluoroethylene are available from Celanese, Millipore, and
Gore Co., respectively. These membranes are naturally hydrophobic
and are not spontaneously wetted by water. Such membranes have
CWSTs ranging from about 28 to about 35 dynes/cm, depending on the
material from which the membrane is made.
The microporous membranes which will be most useful as air filters,
vent filters, and air vents will be those membranes which have as
low a CWST as can be obtained in order to avoid penetration of the
pores of the membrane by liquids with which they come in contact
when in use. Currently the microporous membranes commercially
available which have the lowest CWST are microporous membranes made
of polytetrafluoroethylene, or PTFE. Such membranes are sold by the
Gore Company and by Sumitomo Electric, Incorporated, among others
and are available having a limited number of pore sizes ranging
from 0.05 .mu.m to 1 .mu.m.
The CWST of these PTFE membranes is about 28 dynes/cm, which means
that liquids having surface tensions equal to or lower than this
value will spontaneously wet these membranes. Liquids having
surface tensions higher than 28 dynes/cm will not spontaneously wet
the membranes. Therefore, these PTFE membranes will function
effectively in vents, vent filters, and gas filters as long as the
membrane is not contacted with liquids having surface tensions of
28 dynes/cm or less. However, many aqueous solutions, chemicals,
and many solvents and oils have low surface tensions and will wet
PTFE membranes, either spontaneously or if modest pressure is
accidentally applied. If the surface tension of the liquid is above
the CWST of the membrane, the liquid can be forced to wet the
membrane under pressure. The amount of pressure required is small
if the difference between the surface tension of the liquid and the
CWST of the membrane is small.
A microporous material which has a CWST much less than that of PTFE
membranes would make accessible membranes that could be used in
applications involving a greater variety of chemicals and fluids.
In addition, while PTFE membranes are commercially available, they
are very expensive and are difficult to use in an economical
manner. PTFE membranes are not available having all desired pore
sizes. Furthermore, PTFE is degraded severely by radiation, making
it an undesirable material for use in vents and filters for sterile
medical applications, where sterilization by means of radiation is
the most economical and safe method of sterilizing these products
after manufacture.
It is an object, therefore, of this invention to provide a
microporous membrane which has a CWST controlled to a value
significantly less than that of a membrane made of PTFE and yet
which is above that of certain liquids useful as integrity test
fluids, said membrane being economical to produce and capable of
being made having a wide range of pore sizes in a controlled manner
from materials resistant to damage by high doses of radiation,
particularly doses associated with sterilization.
It is also an object of this invention to provide devices for
processing fluids which use such a hydrophobic membrane to separate
a gas but retain a liquid.
It is a further object to provide methods of using the hydrophobic
membranes of the invention, for example, in gas filtration/drying,
as a venting filter, or as a gas/liquid separator.
SUMMARY OF THE INVENTION
The membranes of this invention are hydrophobic microporous
polymeric membranes having a CWST from less than about 27 dynes/cm.
These membranes are characterized by having a superstrate
fluoropolymer (that is, a polymer formed by the (co)polymerization
of a polymerizable fluorine-containing monomer) permanently
chemically bonded to all the surfaces of a microporous membrane
substrate. For the purposes of this invention, the surface of the
membrane refers not only to the two external, gross surfaces of the
membrane but also to all the internal surfaces of the microporous
structure which would contact a fluid during filtration. Preferred
membranes of this invention are further characterized by having
essentially the same resistance to air flow as the microporous
membrane substrate.
The membranes of the invention are formed by contacting a
microporous polymeric membrane with a solution comprising one or
more polymerizable fluorine-containing monomers and exposing the
membrane to ionizing radiation under conditions which polymerize
the monomer(s) and result in a superstrate hydrophobic
fluoropolymer which is chemically bonded to all the surfaces of the
membrane substrate. By selecting the monomer or combination of
monomers used, the CWST of the product can be controlled to have a
specific value in the desired range.
There are fluoropolymer coatings which are commercially available
which can be used to coat a microporous membrane to impart a low
CWST to its surfaces. Some of these coatings include, for example,
the fluorocarbon coatings FC741 and FC721 sold by the 3M Company.
However, these coatings are not reactive with the membrane and are
not permanent. They can, therefore, be washed off the membrane
during use or during integrity testing. These coatings are also
extremely expensive, with some costing thousands of dollars for one
gallon. Furthermore, certain of these coatings are supplied using
special fluorocarbon solvents which, during application, release
fluorocarbon pollutants harmful to the ozone layer and the general
environment unless expensive pollution control equipment is used,
making the use of such materials impractical. Most important,
however, is the fact that these coatings are not chemically bonded
to the membrane and are fugitive.
The membranes of the present invention are unique in that they can
be produced with a narrowly targeted CWST. These membranes (1) are
not wettable by and therefore not subject to blocking by most
process liquids encountered in important venting applications, (2)
have low resistance to air flow and high vent flow rates, and (3)
are in situ integrity testable by known means.
The membranes of the invention have a resistance to air flow that
is essentially unchanged from that of the substrate membrane before
the superstrate polymer is bonded thereto. This is an indication
that the bonding occurs in an even, uniform way such that the pores
are not significantly constricted by the bonded polymer.
The fluoropolymer is not easily removed, indicating that it is
tightly bonded to the surface. The tightness of this bond can be
tested by exposing the coated membrane to a fluorocarbon liquid
(e.g., trichlorotrifluoroethane), such as is commonly used in
integrity testing. The membrane is exposed to the liquid for
several minutes and the CWST is tested before and after. Merely
coated membranes show a distinct increase in CWST when subjected to
such a test.
The membranes of this invention can conveniently be made by
saturating a preformed microporous membrane with a solution of the
desired polymerizable monomers in a suitable solvent and exposing
the saturated membrane to gamma radiation so as to form a
superstrate fluoropolymer chemically bonded onto all surfaces,
including the pore surfaces and permanently to modify the CWST of
the membrane.
The CWST of the resultant product is to some extent determined by
factors such as the selection of monomers, their concentration,
radiation dose rate, and the nature of the membrane substrate
itself.
BEST MODE OF CARRYING OUT THE INVENTION
The membranes of this invention are prepared from preformed
microporous polymeric membrane substrates. The membranes may be
formed from any material which is a suitable substrate for the
grafting of polymerizable ethylenically unsaturated monomers,
initiated by ionizing radiation. Examples of suitable materials are
polyolefins, polyamides, polyesters, polyurethanes, polysulfones,
and fluoropolymers such as poly(vinylidene fluoride),
polytetrafluoroethylene, perfluoroalkoxy resins, and others. It is
only required that the reactive sites generated by the ionizing
radiation at the polymer surface show sufficient reactivity to
permit formation of a structure in which a polymeric superstrate is
bonded to all the surfaces of the substrate membrane. While
microporous membranes made of any of the above polymer types are
suitable as substrates for this invention, and polyamides are
particularly useful, those membranes which are already hydrophobic
and which have a CWST less than about 35 dynes/cm, for example,
those membranes made from polyolefins and fluoropolymers, are the
more preferred substrates. Especially preferred as substrates are
those membranes made of fluoropolymers. Poly(vinylidene fluoride)
is most preferred as a substrate since it grafts readily and is
stable to radiation.
The microporous membrane substrate is saturated with a solution of
the desired polymerizable, fluorine-containing, ethylenically
unsaturated monomer or monomers. Useful monomers include
perfluoroalkyl acrylates, methacrylates and acrylamides, and other
easily polymerized ethylenically unsaturated molecules containing a
perfluoroalkyl group having a carbon chain from about 4 to about 13
atoms long. Preferred are those fluoroalkanesulfonamidoethyl
acrylates and methacrylates which are available from the 3M Company
under the tradenames FX-13, FX-14, and FX-189, respectively. Most
preferred is the material known as FX-13, which is identified by 3M
as 2-(N-ethylperfluorooctanesulfonamido)ethyl acrylate.
The fluorine-containing monomer may be used alone or in combination
with other fluorine-containing monomers or together with other
polymerizable, (non-fluorine-containing), ethylenically unsaturated
monomers. Such non-fluorine-containing monomers may be both polar
and non-polar and may include unsaturated acids, such as acrylic
and methacrylic acid or their esters, such as hydroxyethyl or
hydroxypropyl acrylate or methacrylate, or other alkyl esters of
these acids derived from alcohols having from about 1 to about 18
carbon atoms. The selection of these comonomers will depend upon
the desired CWST of the product. Thus, if the
fluoroalkyl-containing monomer used alone produces a
surface-modified microporous membrane with a CWST of, for example,
18 dynes/cm, and the desired CWST is 21 dynes/cm, the
perfluoroalkyl monomer can be copolymerized with a copolymerizable
monomer that does not decrease (or even that tends to increase) the
CWST of the substrate polymer. This will tend to modify the effect
of the perfluoroalkyl monomer such that the final CWST of
surface-modified membrane substrate can be precisely
controlled.
In general the use of ionizable monomers such as acrylic or
methacrylic acid with the hydrophobic monomer results in a higher
CWST. Selection of appropriate monomer systems can be guided by
knowledge of reactivity ratios of monomers and their tendency to
bond to polymer substrates using methods known to those skilled in
the art.
The monomers may be dissolved in any suitable solvent or solvent
mixture, as long as the solvent is inert to the polymerization
reaction and will not affect the membrane substrate adversely. For
the purpose of economy and simplified waste disposal, water-based
systems are preferred. If a watermiscible solvent is required to
permit complete dissolution of all the monomers, water-miscible
tertiary alcohols are preferred. Most preferred as a solvent system
is a mixture of 2-methylpropan-2-ol and water containing slightly
more 2-methylpropan-2-ol than is sufficient to bring all components
into solution.
The microporous membrane substrate is then saturated with the
monomer solution by any appropriate means. Flat sheets of membrane
may be dipped in a bath of solution, whereas continuous lengths of
membrane may be saturated by known means of wet treatment of
continuous, porous webs. For example, a continuous length of
membrane may be passed through a bath containing the monomer
solution, or it may be passed over a vacuum suction drum and
monomer solution can be drawn through the membrane. Alternatively,
an entire roll of continuous microporous membrane may be immersed
in a vessel of monomer solution until fully and uniformly saturated
with the solution.
Regardless of the manner in which a continuous length of membrane
is saturated with the monomer solution, it is exposed to ionizing
radiation. The preferred means for doing so is to interleave the
saturated web with a porous non-woven web. (If the membrane had
been saturated in roll form already interleaved in this fashion
then re-rolling is not necessary.) The interleaved roll is then
placed in a container (preferably a stainless steel canister)
containing excess monomer solution which maintains the roll in
contact with liquid monomer solution during exposure to radiation.
Any source of ionizing radiation can be used that is capable of
initiating polymerization but a preferred source is a .sup.60 Co
source. Any irradiation dose rate is acceptable provided that it
yields a modified CWST with the desired surface properties and that
the membrane substrate is not damaged by the radiation. Dose rates
from about 5 to about 100 kilorads/hr and preferably from about 5
to about 70 kilorads/hr have been found effective. It is sometimes
found that higher radiation rates in the broader range have the
unexpected result of yielding membranes having a higher CWST than
membranes prepared similarly but using a lower dose rate. While not
wanting to be bound by any particular theory, it is believed that
the higher CWST results from a decreased amount of grafting because
the higher radiation rate promotes side reactions such as the
formation of homopolymers of the polymerizing monomers which are
not bonded to the substrate membrane. A dose rate of about 10
kilorads/hr and a total dose of 0.2 Mrads is preferred for grafting
to membrane substrates made of poly(vinylidene fluoride).
After irradiation, the roll of membrane is preferably washed with
water to remove polymeric debris that is not bonded to the
substrate. Any means of washing which causes a flow of water
tangential to the membrane and generally perpendicular to the
length of the web is effective. Particularly effective is passing
water tangentially through an interleaved roll or irradiated
membrane.
Debris, which is usually a polymer of the polymerizing monomer(s),
is often present, along with the surface-modified substrate, in the
form of hard gel particles which can adhere to the membrane.
Incorporation of a minor proportion of a polar monomer such as
acrylic acid, methacrylic acid, or hydroxypropyl acrylate makes
this debris more easily washed away by water.
After washing, the membrane may be dried by conventional means such
as tunnel ovens or hot drum driers. Alternatively, it may be stored
wet or processed further, depending on its end use.
The preparation and evaluation of microporous membranes having a
CWST substantially lower than that of PTFE membranes is described
below.
GENERAL PROCEDURE FOR MEASURING CRITICAL WETTING SURFACE TENSION
(CWST)
The CWST of microporous membranes was determined by testing the
membrane for its ability to be wetted by a series of pure normal
paraffin liquids having known surface tensions. The liquids used in
this test were:
______________________________________ Surface Tension.sup.(a)
Liquid dynes/cm ______________________________________ n-Hexane 18
n-Heptane 20 n-Octane 21 n-Nonane 22 n-Decane 23 n-Undecane 24
n-Dodecane 25 Tetradecane 26 n-Hexadecane 27
______________________________________ .sup.(a) Surface tension at
25.degree. C. estimated from J. Phys. Chem. Ref. Data, Vol. l, No.
4, 1972.
Normal paraffins having surface tensions significantly higher than
those above are not liquid at room temperature. Therefore, to
estimate CWSTs above 26 dynes/cm, the following non-hydrocarbon
liquids were used as test liquids:
______________________________________ acetonitrile 29 dynes/cm 12%
by weight tertiary 30 dynes/cm butyl alcohol in H.sub.2 O 10% by
weight tertiary 33 dynes/cm. butyl alcohol in H.sub.2 O 3% by
weight tertiary 35 dynes/cm butyl alcohol in H.sub.2 O
______________________________________
A drop of each liquid was placed gently on the surface of the
membrane tested using a glass pipet, starting with the liquid
having the lowest surface tension. If the liquid wetted the
membrane the membrane would be tested with the liquid having the
next higher surface tension. This sequence was repeated until a
liquid was found which did not wet the membrane. The critical
wetting surface tension was defined to be the mean (rounded down to
the nearest dyne/cm) of the surface tension of the liquid having
the highest surface tension which wetted the membrane and the
surface tension of the liquid having the lowest surface tension
which did not wet the membrane.
GENERAL PROCEDURE FOR MEASURING AIR FLOW RESISTANCE
For the purposes of this disclosure the resistance to air flow was
measured as the pressure drop across two layers of membrane
required to cause a flow of air through the two layers at a face
velocity of 7 ft/min at atmospheric pressure. This was measured
using an apparatus built for this purpose. In this test two layers
of membrane were held against a wire screen support and the side of
the membrane away from the screen was pressurized with air until an
air flow of 7 ft/min was attained. The region downstream of the
support screen was open to the atmosphere. The pressure upstream of
the membranes was measured and reported as the air flow
resistance.
EXAMPLE 1
A dry, microporous poly(vinylidene fluoride) membrane manufactured
by Pall Corporation, sold under the trademark Emflon.TM. II, and
having a pore size of 0.2 .mu.m was immersed in a solution of 0.50
weight percent (based on the solvent weight) of FX-13 (a product of
the 3M Company identified as
2-(N-ethylperfluorooctanesulfonamido)ethyl acrylate) in a mixture
of 45% by weight tertiary butyl alcohol and 55% by weight water.
The membrane was saturated with this solution. While being immersed
in this liquid the membrane was exposed to gamma radiation from a
.sup.60 Co source at a dose rate of 10 kilorads/hr for 20 hours.
After being irradiated the membrane was removed from the solution
and rinsed off with running deionized water and dried in an air
oven at 100.degree. C. for 10 minutes.
The CWST and air flow resistance of the dried membrane was measured
according to the General Procedures above. The CWST of the membrane
was found to be 21 dynes/cm compared with 34 dynes/cm measured for
an untreated poly(vinylidene fluoride) membrane. The air flow
resistance of the membrane was found to be 1.7 in. Hg, the same as
that of an untreated membrane. The CWST and air flow resistance of
the membrane of this Example and the untreated membrane (referred
to as Control) are summarized below in Table I.
EXAMPLE 2
A dry, microporous poly(vinylidene fluoride) membrane manufactured
by Pall Corporation and sold under the trademark Emflon.TM. II
having a pore size of 0.2 .mu.m was treated as in Example 1, except
that the concentration of FX-13 in the solution was 0.10 weight
percent.
The dry membrane was found to have a CWST of 21 dynes/cm, much
lower than that of the Control, and an air flow resistance of 1.6
in. Hg, essentially unchanged from that of the Control. This
information is summarized in Table I.
EXAMPLE 3
A dry, microporous poly(vinylidene fluoride) membrane manufactured
by Pall Corporation and sold under the trademark Emflon.TM. II
having a pore size of 0.2 .mu.m was treated as in Example 1, except
that the concentration of FX-13 in the solution was 0.05 weight
percent.
When the membrane was dry it was found to have a CWST of 24
dynes/cm, much lower than that of the untreated Control and also
lower than that of Poreflon, a commercial, microporous PTFE
membrane available from Sumitomo Electric having the same pore
size. Its air flow resistance remained essentially unchanged from
that of the Control. This information is summarized in Table I.
EXAMPLE 4
A dry, microporous poly(vinylidene fluoride) membrane manufactured
by Pall Corporation and sold under the trademark Emflon.TM. II
having a pore size of 0.2 .mu.m was treated as in Example 1, except
that the concentration of FX-13 in the solution was 0.01 weight
percent.
When the membrane was dry it was found to have a CWST of 34
dynes/cm, about the same as that of the untreated Control and
higher than that of a commercially available PTFE membrane having
the same pore size. Its air flow resistance remained essentially
unchanged from that of the Control. This information is summarized
in Table 1.
The results in Table I show that treatment of poly(vinylidene
fluoride) membranes according to the method of Example 1 using
concentrations of FX-13 ranging from 0.05% to 0.50% by weight
yielded microporous membranes having a CWST of 24 dynes/cm and
less. These membranes were not wetted by liquids having surface
tensions ranging from 25 dynes/cm to 27 dynes/cm, whereas these
liquids did spontaneously wet both the Poreflon PTFE membrane and
the untreated Control. The results in Table I also demonstrate that
the air flow resistance of the treated membranes remained
essentially unchanged from that of the Control.
TABLE I ______________________________________ Air Flow Membrane of
CWST Resistance Example FX-13 (%) (dynes/cm) (in. Hg)
______________________________________ 1 0.50 21 1.7 2 0.10 21 1.6
3 0.05 24 1.6 4 0.01 34 1.6 Control -- 34 1.6 Poreflon -- 28 N/A
______________________________________
Examples 5-8 demonstrate that Certain comonomers may be added to
the FX-13 treatment solution for the purpose of controlling the
CWST of the product.
EXAMPLE 5
A dry, microporous poly(vinylidene fluoride) membrane manufactured
by Pall Corporation and sold under the trademark Emflon.TM. II
having a pore size of 0.2 .mu.m was treated as in Example 1, except
that the concentration of FX-13 in the solution was 0.15 weight
percent.
When the membrane was dry, it was found to have a CWST of 22
dynes/cm, much lower than that of an untreated membrane (Control)
and an air flow resistance of 1.7 in. Hg, essentially unchanged
from that of an untreated Control. This information is summarized
in Table II below.
EXAMPLE 6
A dry, microporous poly(vinylidene fluoride) membrane manufactured
by Pall Corporation and sold under the trademark Emflon.TM. II
having a pore size of 0.2 .mu.m was treated as in Example 5, except
that the treating solution further contained 0.05 weight percent
methacrylic acid.
The dried product membrane was found to have a CWST of 22 dynes/cm,
equal to that of the membrane of Example 5, and an air flow
resistance of 1.8 in. Hg. This information is summarized in Table
II below.
EXAMPLE 7
A dry, microporous poly(vinylidene fluoride) membrane manufactured
by Pall Corporation and sold under the trademark Emflon.TM. II
having a pore size of 0.2 .mu.m was treated as in Example 5, except
that the treating solution further contained 0.10 weight percent
methacrylic acid.
The resultant membrane was found to have a CWST of 24 dynes/cm,
slightly higher than that of the membrane of Example 5, but still
significantly below that of Poreflon, a commercial PTFE membrane.
Its air flow resistance was 1.8 in. Hg. This information is
summarized in Table II below.
EXAMPLE 8
A dry, microporous poly(vinylidene fluoride) membrane manufactured
by Pall Corporation and sold under the trademark Emflon.TM. II
having a pore size of 0.2 .mu.m was treated as in Example 5, except
that the treating solution further contained 0.20 weight percent
methacrylic acid.
The resultant membrane was found to have a CWST of 29 dynes/cm,
significantly below that of an untreated membrane and just above
that of a commercial PTFE membrane. Its air flow resistance was 1.8
in. Hg, slightly higher than that of the membrane of Example 5.
This information is summarized in Table II below.
The results in Table II show that using a polar comonomer together
with FX-13 leads to a product having a higher CWST than if the
polar comonomer had not been used. The results further show that
under the conditions used in Examples 6-8 as the amount of
methacrylic acid used is increased to about 0.20 weight percent the
CWST of the product is increased to that of a Poreflon PTFE
membrane. The results in Table II also show that a membrane having
a CWST as low as 19 dynes/cm can be obtained by using 2-ethylhexyl
methacrylate together with FX-13.
Examples 9 and 10 demonstrate that the intensity of radiation used
to effect the surface modification influences the CWST of the
resultant membrane.
EXAMPLE 9
A dry, microporous poly(vinylidene fluoride) membrane manufactured
by Pall Corporation and sold under the trademark Emflon.TM. II
having a pore size of 0.2 .mu.m was treated as in Example 1, except
that the treating solution further contained 0.05 weight percent
2-ethylhexyl methacrylate.
The dried product membrane was found to have a CWST of 19 dynes/cm
and an air flow resistance was 1.7 in. Hg. This is summarized in
Table II below.
TABLE II ______________________________________ Ethyl Mem- Hexyl
brane Meth- meth- CWST Air Flow of FX-13 acrylic acrylate (dynes/
Resistance Example (%) (%) (%) cm) (in. Hg)
______________________________________ 5 0.15 0 0 22 1.7 6 0.15
0.05 0 22 1.8 7 0.15 0.10 0 24 1.8 8 0.15 0.20 0 29 1.8 1 0.50 0 0
21 1.7 9 0.50 0 0.05 19 1.7 Control -- -- -- 34 1.6 Poreflon -- --
-- 28 N/A ______________________________________
EXAMPLE 10
A dry, microporous poly(vinylidene fluoride) membrane manufactured
by Pall Corporation and sold under the trademark Emflon.TM. II
having a pore size of 0.2 .mu.m was treated as in Example 3, except
that the dose Rate of the irradiation was 50 kilorads/hr.
The resultant membrane was found to have a CWST of 28 dynes/cm,
higher than that of a membrane treated identically but irradiated
at a dose rate of 10 kilorads/hr and about the same as the CWST of
Poreflon, a commercial PTFE membrane. Its air flow resistance was
1.6 in. Hg, the same as that of an untreated membrane (Control).
This is summarized in Table III below.
EXAMPLE 11
A dry, microporous poly(vinylidene fluoride) membrane manufactured
by Pall Corporation and sold under the trademark Emflon.TM. II
having a pore size of 0.2 .mu.m was treated as in Example 2, except
that the dose rate of the radiation was 50 kilorads/hr.
The resultant membrane was found to have a CWST of 24 dynes/cm,
somewhat higher than that of a membrane treated identically but
irradiated at a dose rate of 10 kilorads/hr yet still significantly
lower than the CWST of a PTFE membrane. Its air flow resistance was
1.6 in. Hg, the same as that of an untreated membrane (Control).
This is summarized in Table III below.
As can be seen in Table III, in each case that irradiation was
performed at a dose rate of 50 kilorads/hr the resultant CWST of
the product was higher than that obtained at a dose rate of 10
kilorads/hr.
TABLE III ______________________________________ Membrane Dose Air
Flow of Rate CWST Resistance Example FX-13 (%) (krd/hr) (dynes/cm)
(in. Hg) ______________________________________ 3 0.05 10 24 1.6 10
0.05 50 28 1.6 2 0.10 10 21 1.6 11 0.10 50 24 1.6 Control -- -- 34
1.6 Poreflon -- -- 28 N/A
______________________________________
Example 12 illustrates how a very low CWST membrane can be prepared
directly from an undried membrane substrate, still wet from the
membrane-forming process.
EXAMPLE 12
A dry, microporous poly(vinylidene fluoride) membrane having a pore
size of 0.1 .mu.m and having a non-woven polypropylene internal
support was prepared by conventional means, and all adjuvant
materials were washed from the membrane using water. The water-wet
membrane was then treated as described in Example 1.
The resultant membrane was found to have a CWST of 22 dynes/cm,
significantly lower than that of a commercial PTFE membrane and
much lower than that of an untreated membrane of the same type
which was dried in the same manner as the membrane of this Example.
The untreated membrane is referred to as Control 12 to distinguish
it from the Control referred to in previous Examples. The air flow
resistance of the treated membrane of this Example was 9.0 in. Hg,
unchanged from that of Control 12. This information is summarized
in Table IV below.
TABLE IV ______________________________________ Membrane Air Flow
of CWST Resistance Example (dynes/cm) (in. Hg)
______________________________________ 12 22 9.0 Control 12 34 9.0
Poreflon 28 N/A ______________________________________
EXAMPLE 13 (COMPARATIVE)
This Example describes the preparation of a conventional coated
membrane and the integrity of this coating, for the purposes of
comparison with a grafted membrane according to the invention. A
dry, microporous poly(vinylidene fluoride) membrane manufactured by
Pall Corporation and sold under the trademark Emflon.TM. II having
a pore size of 0.2 .mu.m was agitated gently for 5 minutes in a
solution containing 0.5% by weight in a mixture of fluorocarbon
solvents. The solution was prepared by diluting one part by volume
of FC721 (a commercial fluorocarbon coating available from the 3M
Company as a 2% by weight solution of a fluoropolymer composition
in at least one fluorocarbon solvent) with 3 parts by volume Freon
TF (a trichlorotrifluoroethane product of E. I. DuPont de Nemours,
Inc.). The membrane was then removed from the solution and dried in
an air oven at 100.degree. C. for 10 minutes. The treated membrane
was found to have a CWST of 22 dynes/cm.
The membrane of Comparative Example 13 was agitated gently for a
total of 3 minutes in three successive portions of Freon TF, a
liquid commonly used to integrity test filters containing
hydrophobic filter membranes. After removal from the Freon, the
membrane was dried in an air oven for 4 minutes at 100.degree. C.
The Freon TF-exposed membrane had a CWST of 30 dynes/cm, much
higher than the CWST before exposure to the Freon TF and even
higher than the CWST of Poreflon, a commercial microporous PTFE
membrane.
The membrane of Example 1 was exposed to Freon TF for 3 minutes and
dried in a similar fashion. After drying the CWST remained 21
dynes/cm, unchanged from its value prior to exposure to Freon
TF.
The above results are summarized in Table V below. These results
show that, after brief exposure to Freon TF, a hydrophobic membrane
coated by methods previously known to the skilled artisan is no
longer as hydrophobic as it was prior to Freon TF exposure, in
fact, no longer as hydrophobic as a PTFE membrane. By contrast the
membrane of the present invention retains its hydrophobicity upon
exposure to Freon.
TABLE V ______________________________________ Membrane CWST CWST
of Before Freon After Freon Example Exposure Exposure
______________________________________ 13 (Comparative) 22 dynes/cm
29 dynes/cm 1 21 dynes/cm 21 dynes/cm
______________________________________
EXAMPLE 14
A commercial PTFE membrane having a pore size of 0.2 .mu.m
(Poreflon, a product of Sumitomo Electric, Inc.) was treated in the
manner described in Example 1, except that the concentration of
FX-13 in the solution was 2.0% by weight and the solvent was a
mixture of 55% by weight tertiary butyl alcohol and 45% by weight
water.
The resultant membrane had a CWST of 19 dynes/cm, significantly
lower than that of the untreated PTFE membrane, designated
"Poreflon Control" in Table VI. The air flow resistance of the
treated sample was measured to be 1.2 in. Hg, slightly lower than
that of the Poreflon Control. This information is summarized in
Table VI below.
The data in Table VI show that a PTFE membrane can be made even
more hydrophobic, i.e., its CWST can be made lower, by treatment in
accordance with the present invention.
TABLE VI ______________________________________ Membrane Air Flow
of CWST Resistance Example (dynes/cm) (in. Hg)
______________________________________ 14 19 1.3 Poreflon Control
28 1.5 ______________________________________
* * * * *